Continuous sonic wave analyzer

ABSTRACT

A gas analyzer uses continuous sonic signals through a conduit to determine the composition of a gas in the conduit. A transmitting transducer drives sonic signals at a fixed frequency and a second transducer receives the sonic signals. The phase shift between two signals corresponds to the speed of sound through the gas and is related to the composition of the gas. The electronic versions of these signals are processed by lowering, or dividing, the fixed frequency which expands the range of phase shift measurement and allows the determination of an expanded range for the gas composition. In an ozone generation system, the gas analyzer is highly suitable for determining the composition of gases derived from air as a gas of known composition and a calibration point.

BACKGROUND OF THE INVENTION

This invention relates to gas composition analyzers and, moreparticularly, to gas composition analyzers for the generation of ozone.Ozone is a highly active form of oxygen often used for disinfection andwater treatment. Due to its characteristics, ozone is typicallygenerated on site and at the time of use.

Ozone may be generated in many ways, one of which is by the ionizationof oxygen using electrical discharge to create a plasma. Ozone whengenerated by electrical discharge has a concentration that depends onmany factors, including but not exclusively, the composition of the feedgas, the flow rate of feed gas, the temperature of ozone generationcell, the dimensions and materials of the cell, and the electrical powerused to generate the plasma. The plurality of factors affecting ozoneproduction makes it very difficult to predict ozone production with anyprecision. If control or knowledge of ozone production is desired, it isnecessary or desirable to monitor ozone production. An analyzer on siteis required for this purpose.

There are several different techniques available to an analyzer formeasuring the concentration of ozone in a gas. These include using theabsorption of UV light in the gas, such as found in the products fromOxidation Technologies, LLC of Inwood, Iowa and Teledyne API of SanDiego, Calif. This technique is effective but production costs are high.Furthermore, no information on the composition of the feed gas to theozone-generating cell is obtained. Knowledge of the feed gascomposition, which may consist of dry air with an increasedconcentration of oxygen, is desirable. Electrical discharge ozonegenerators operate more efficiently with a high proportion of oxygen.Therefore, oxygen concentrators are sometimes used to increase oxygenfrom 20.9% (ambient air) to values above 90%. For an electricaldischarge ozone generator, the presence of small amounts of nitrogen inthe feed gas appears to enhance efficiency significantly. But it ispossible to remove too much nitrogen from the feed gas such thatefficiency of the cell is reduced. In such oxygen-concentrated air, theprincipal components are nitrogen, oxygen, and a small amount of argon.By complementation, the concentration of nitrogen can be estimated fromthe concentration of oxygen.

Use of the speed of sound to estimate the concentration of ozone in agas is described in U.S. Pat. No. 5,644,070 (Gibboney). With thetemperature of the feed gas, the speed of sound of the feed gas, thetemperature of the gas as it emerges from an ozone generator, and thespeed of sound of the gas as it emerges from the ozone generatormeasured or known, the speed of a sound pulse in the gas is determinedby measuring delay over a known path length. The four measured or knownvariables are used to estimate the concentration of ozone. However, witha resonant transducer a pulse necessarily consists of multiple cycleswhich make the precise determination of the arrival of a pulse of sounddifficult; it is difficult to ascertain when a pulse begins and when itends. A further disadvantage is that the described system is complex.The sound pulses require relatively long measurement paths and henceconduits with relatively high volume which increases the required samplegas volumes. A scavenging pump, which is costly, is used to move eitherthe feed gas to the ozone generation cell or the output gas from thecell. This complicates the measurement system. The pump must be made ofmaterials that do not deteriorate over time in the presence of highconcentrations of corrosive ozone.

The speed of sound in a continuous sonic wave is used to help determinethe concentrations of two gases, neither of them ozone. U.S. Pat. Nos.6,202,468 and 6,520,001 by the present inventor describe a system inwhich that technique is combined with another. Two distinct andunrelated physical parameters, paramagnetism and the speed of sound, aremeasured to determine the concentration of both oxygen and carbondioxide in respiratory gas. In this case, the use of sound alone cannotdetermine the concentration of either gas.

Hence there is a need for a low cost analyzer with the capability tomeasure both the concentration of oxygen in feed gas to an ozonegenerating cell and the concentration of ozone in the cell output. Suchan instrument may be used for the assessment of generated ozone and forprocess control. For example, oxygen concentration may be adjusted,based upon instrument output, so as to maintain a desired concentrationof ozone, and cell power may be controlled, based instrument output tomaintain a desired concentration of ozone. A single low-cost analyzer tohandle both of these functions would be both convenient and economical.It is an object of the present invention to perform both functions in asingle, reliable, low-cost instrument.

BRIEF SUMMARY OF THE INVENTION

The present invention provides for an analyzer for one or more gasesderived from a first gas of known composition and speed of sound, eachderived gas having a concentration of a component changed. The analyzerhas: a first transducer which drives continuous sound waves responsiveto a fixed frequency signal source; a conduit acoustically connected tothe first transducer and selectively receiving and holding samples ofthe first gas and one or more derived gases; a second transduceracoustically connected to the conduit opposite the first transducerunit, the second transducer which receives sound waves from the firsttransducer through the conduit and generates second transducer signalsresponsive to the received sound waves; a processing unit which receivesthe fixed frequency signal source signals and the second transducersignals, and which determines a relative phase shift between thefrequency source signals and second transducer signals for a gas samplein the conduit, the relative phase shift corresponding to a differenceof speed of sound in one gas sample relative to another gas sample, theprocessing unit including circuitry lowering the frequency of thereceived fixed frequency source signals and second transducer signals toexpand the range of measurement of the relative phase shift; and acalculating unit which determines from the first gas of knowncomposition the speed of sound of the one or more gases derived from thefirst gas, and which calculates the composition of a sample of one ormore derived gases from the first gas as a reference.

The present invention also provides for a method of operating ananalyzer for one or more gases derived from a first gas of knowncomposition and speed of sound, each derived gas having a concentrationof a component changed. The method has the steps of: driving continuoussound waves with a first transducer in response to fixed frequencyelectrical signals through a conduit holding a sample of the first gasor one or more derived gases at a time; receiving the sound waves driventhrough the conduit by a second transducer and generating electricalsignals in response to the received sound waves, a relative phase shiftbetween the received sound wave signals and the driven sound wavesignals corresponding to a relative speed of sound in the gas samples;processing the fixed frequency electrical signals and the electricalsignals generated signals in response to the received sound waves at alowered frequency to expand the range of measurement of the relativephase shift; determining the speed of sound of the first gas of knowncomposition and one or more gases derived from the first gas in theexpanded range from the relative phase shift of gas samples of the firstgas of known composition and one or more gases derived from the firstgas in the conduit; and calculating a composition of a sample of one ormore gases derived from the first gas as a reference.

The present invention further provides for a method of determining thecomposition of one or more gases derived from a first gas of knowncomposition and speed of sound. The method has the steps of: drivingcontinuous sonic waves through a conduit at a fixed frequency, a phasedifference between sonic waves entering the conduit and leaving theconduit corresponding to a speed of sound of a gas in the conduit;processing electronic signals corresponding to the continuous sonicwaves entering the conduit and leaving the conduit at a loweredfrequency to expand the range of measurement of the phase shift;changing the gas in the conduit among the first gas and the one or morederived gases; determining the speed of sound of the first gas of knowncomposition and the one or more gases derived from the first gas in theexpanded range from a relative phase shift of gases of the first gas ofknown composition and the one or more derived gases, the relative phaseshift corresponding to a difference of speed of sound in one gas samplerelative to another gas sample; and calculating a composition of the oneor more derived gases from the first gas as a reference.

Other objects, features, and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionand the accompanying drawings, in which like reference designationsrepresent like features throughout the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the general organization of an analyzer in an ozonegeneration system according to one embodiment of the present invention.

FIG. 2 represents fixed frequency signals to the source transducer andthe signals from the receiving transducer of the analyzer unit of FIG. 1.

FIG. 3 illustrates how lowered frequency of the FIG. 2 signals to thesource transducer and from the receiving transducer of the analyzer unitexpands the range of measurement of phase shift, according to anembodiment of the present invention.

FIG. 4A shows a simplified circuit for the phase shift detector in FIG.1 ; FIG. 4B shows exemplary transmitting transducer signals andreceiving transducer signals, the resulting output signal from the FIG.4A phase shift detector, and resulting signal after the phase detectoroutput has passed through a low-pass filter of FIG. 1 ; FIG. 4C showsthe output of the low-pass filter for different phase shifts; and FIG.4D shows the output of the low-pass filter for different phase shiftsfor the baseline adjustment.

FIG. 5 shows a flow chart of operations of the analyzer unit in theoperation of ozone generation system according to an embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

As described below, the present invention provides for the measurementof ozone concentration with high resolution and precision. The ozoneconcentration is measured relatively independently of the oxygenconcentration in the feed gas and independently of temperature. Theconstruction of the analyzer is also simple and low-cost.

FIG. 1 shows a generalized view of a portion of an ozone generationsystem with a continuous sonic wave analyzer unit according to oneembodiment of the present invention. It should be noted that drawing isrepresentational and the elements of the drawing are not drawn to scale.The system has an ozone generation block 300 and a continuous sonic waveanalyzer unit 100 which is formed by a transducer/valve block 400 andcontroller/analysis block 200. The ozone generation block 300 receivescompressed air from a source (not shown) and delivers generated gasincluding ozone to a process, i.e., the particular application of theozone. The compressed air (gas flow is shown by the broadened arrows inthe drawing) is received by a concentrator 301 which increases thepercentage of oxygen in the resulting gas. The gas from the concentrator301, which may be a swing pressure absorption device, is passed to anozone generator 302, typically an electrical discharge cell. The gasoutput from the ozone generator 302 is sent to an inlet pressureregulator 303 which controls the pressure of the gas and ozone sent tothe process, the application using the generated ozone.

The analyzer unit 100 determines the relative speeds of sound of gas atdifferent locations of the ozone generation block 300 and comprises atransducer/valve block 400 and a controller/analysis block 200. Thetransducer/valve block 400 processes samples of gas from the differentlocations and the controller/analysis block 200 controls the operationsof the transducer/valve block 400 and analyzes the output from thetransducer/valve block 400. The transducer/valve block 400 has a firstinput valve 403 which receives the compressed air from the source to theconcentrator 301; a second input valve 402 which receives the output gasfrom the concentrator 301 to the ozone generator 302; and a third inputvalve 401 which receives the output gas from the ozone generator 302 tothe inlet pressure regulator 303. The outputs of the valves 401-403 areconnected to a first transducer unit 404 which has its output connectedto a gas conduit 405 which in turn is connected to a second transducerunit 406. The transducer unit 404 transmits sound in a continuous wavethrough the conduit 405 to the receiving transducer unit 406 todetermine the relative speed of sound through the gas in the conduit 405(and transmitting transducer units 404 and receiving transducer unit406). The gas output of the receiving transducer unit 406 is connectedto a destruct unit 407 which eliminates the ozone in the sampled gasbefore releasing the gas into the ambient air.

In general, a reference phase reading is taken for a first gas of knowncomposition (usually ambient air) and then unknown second and thirdgases are introduced, producing corresponding changes of phase shift,and from these changes of phase shift and the known speed of sound ofthe first gas, the speed of sound of the second and third gases arecomputed.

Ozone is highly corrosive and care is taken in the selection of thecomponents in contact with ozone. The transducer units 404 and 406 areformed from aluminum which forms a tough coating of aluminum oxide andthe conduit 405 is formed from polytetrafluoroethylene (PTFE) tubingwhich resists ozone. The conduit 405 is also temperature-controlled tomaintain the temperature of the gas in the conduit at a desiredtemperature and has a relatively large thermal mass to stabilize theconduit temperature. The length L of conduit 405 is preferably longenough to give good sensitivity to the device and to prevent artifactsdue to standing waves, yet short enough to be low in volume, convenientto fabricate, and unambiguous with respect to measuring phase shift.From the point of view of sensitivity and standing waves, a preferablepathlength L may be about 24 wavelengths, although other pathlengths maybe used. But a pathlength of 24 wavelengths may result in ambiguousreadings due to excessive change of phase shift as speed of sound varieswith changing gas composition. For example, phase shift due to replacingair with oxygen would be about 0.9 wavelengths. Phase shift due toreplacing air with a mixture of ozone and oxygen may be as much as twowavelengths. A method to mitigate this problem using frequency divisionis a feature of the invention and is described below. The method allowschoice of pathlength based on a desired compromise of the mentionedfactors without concern for ambiguous results from a phase detector.

The controller block 200 of the analyzer unit 100 has an oscillator unit201 which drives the first transducer unit 404 in the transducer/valveblock 400. A first counter 205 receives a control signal from thecontrol unit 208 and a driving signal from the oscillator 201. Anamplifier 202 in the controller block 200 receives the electrical outputof the second transducer unit 406. The output of the amplifier 202 isreceived by a comparator 203 which shapes the amplified signals tosquare waves. The comparator output is sent to a second counter 204which counter 204 also receives a control signal from the control unit208. The outputs of both counters 204 and 205 are sent to a phasedetector 206 and the phase detector output is passed through a low-passfilter 207 to the control unit 208.

The present invention uses the speed of sound in a gas to determine theconcentration of ozone in the gas. Sound in ozone is considerably slowerthan sound in oxygen due to higher molecular weight of ozone relative tooxygen. Likewise, sound in oxygen is slower than sound in air, due tohigher average molecular weight of oxygen relative to air. For a gaswhich has only two components and if the speed of sound of eachcomponent is distinct from the other, the measured speed of sound thoughthe gas is characterized by the proportions of the two components. Thisis true even if the components themselves comprise mixtures of more thanone gas. Discussion of this may be found in the previously cited U.S.Pat. Nos. 6,202,468 and 6,520,001.

The speed of sound in a gas is found by the known characteristics of thefirst gas introduced, and the corresponding change in phase shift whichthe continuous sonic wave originating from the source transducerundergoes as the wave travels to the receiving transducer withsubsequent gases. FIG. 2 shows the sonic wave train from thetransmitting transducer 404 as a series of square waves indicating thedigital nature of the signals and the circuitry processing the signals.Analog signals and circuitry may also be used. The rising edges of thewaves shown as solid vertical bars to serve as reference points to aidthe reader's understanding. Likewise, another series of square wavesreceived by the second transducer 405 is illustrated with solid barrising edge of each sonic waves received by the second transducer 405.An arrow indicates the phase shift reflecting the time interval for aparticular wave front to travel from the source transducer 404 to thereceiving transducer 405. It should be evident that the longer the timeinterval, or the slower the speed of the sonic waves through the gasmedium, the greater the phase shift.

The phase shift should be kept within a restricted range due to thenature of the continuous wave to determine the amount of shift withcertainty. For example, it is difficult to determine whether a phaseshift is x or x+i*360°, where i is an integer. So in this example onlyphase shifts of less than 360° should be undertaken. But this severelylimits the range of speeds which can be determined. It should be notedthat limited phase range is also dependent upon the circuits used todetermine the phase shift which may limit the phase shift even more,such as from 0° to 180°. In any case, the present invention expands therange of speeds which can be determined as explained below.

While the source transducer is driven at a fixed frequency, thefrequency of the source signals is lowered for processing. FIG. 3represents an exemplary lowering of frequency of 66% or stateddifferently, dividing the frequency by 3, by the elimination of 2 out of3 vertical bars. Elimination is indicated by the replacement of a solidbar (representing a rising edge) with a barred bar. With the lowering ofthe frequency, the phase shift range (and hence the sound speed range)which can be determined is accordingly expanded. That is, assuming aphase limitation of 0-360°, the frequency lowering by 3 allows the phaseshift range to be expanded by 3 so that the range is expanded to0-1080°. Frequency reduction proportionately increases the range ofphase shift measurement and allows high precision measurement withoutthe shortcomings of a short path length conduit. A short path length maytypically introduce signal artifact due to standing waves or residues ofstanding waves in the conduit. Frequency reduction avoids theseproblems.

A simple representation of the electronic circuitry in FIG. 4Ademonstrates the measurement of phase shifts as previously described.Two square wave data streams, A representing the signals for thetransmitting transducer (404 in FIG. 1 ) and B representing the signalsfor the receiving transducer (406 in FIG. 1 ), are input to anExclusive-OR logic gate (part of the circuitry of phase detector 206).The output C of the gate is input to a low-pass filter (207 in FIG. 1 )which has an output D. FIG. 4B shows the relationship of the A and Bsignals, and the output C signal of the Exclusive-OR gate. Besides anExclusive-OR gate, an Exclusive-NOR gate may also used for the phasedetector 206.

FIG. 4B also shows the output D signal of the low-pass filter 207 bywhich the mostly varying signal of the Exclusive-OR gate is filtered toreflect the “average” value of the output signal. For example, if the Aand B signals are completely out of phase with each other (i.e., a phasedifference of 180°), then the filtered phase difference, or phase shift,signal, the output D, is a maximum; if the A and B signals arecompletely in phase with each other (i.e., a phase difference of 0°),then the output D is a minimum, i.e., zero. If the A and B signals are“half” out of phase, or half in phase, i.e., the phase difference is90°, and the output D is halfway between the maximum and the minimum,i.e., one-half the maximum. FIG. 4C illustrates how this phase detectorvalue, the output D, varies with the phase difference between signals Aand B. Here the phase differences are shown as values less than −540° togreater than 540°. The minus and positive values indicate whether the A,or the B, signals leads the B, or the A, signals. As described above,the A signals lag the B signals. FIG. 4C further graphically shows whythe phase shift should be within a restricted range. In thisillustration the phase shift should be restricted to a range of 180° toavoid ambiguity in determining the phase shift from an output D value.

When the frequency of A and B signals is lowered, the output D of thelow-pass filter 207 is changed. In the example of FIG. 4D counters (205and 204 in FIG. 1 ) are inserted into the data streams of the A and Bdigital signals to lower the frequency by a divisor of 8 (N=8 for thecounters). The divisor N is arbitrary and chosen to suit the convenienceof the designer. The output D is correspondingly spread. Instead of acycle of 360° (as shown in FIG. 4C), the output D cycle is 8 timeslarger, 2880°, i.e., the output D signal repeats every 2880°. In thisexample, the phase shift is expanded to a range of 1440°, one-half of2880°. This is a far larger range than 180° restriction without thefrequency-lowered signals.

Thus an expanded range for the filtered phase shift signal, the outputD, is easily implemented by digital counters, e.g., counters 205 and 204in FIG. 1 . Using the example above where N=8, the counter for the Asignal data stream is set to zero and then started. When the A counterreaches a particular value, say 1, then the counter for the B signaldata stream is set to zero and started. This assures a non-negativestarting value for the phase detector where N=8. The output D is thusbetween zero and 0.125 (assuming the maximum output D is 1.0). Theamount 0.125 is ⅛. If the phase difference between the A and B signals,due to a different gas sample in the conduit 405 (see FIG. 1 ), nowincreases, the output D also increases linearly over a range of 1260°(1440°*(1−(⅛)) up to the maximum value. Again it should be noted thatthe counter divisor N and corresponding expansion of the phase shiftmeasurement is arbitrary.

The two frequency-lowering counters 205 and 204 introduce phaseuncertainty. In the above case of divide-by-8, there are 8 possiblephase relationships depending on the counting relationship of the twocounters. If the first counter 205 has count N, then the second counter204 may have any of (N+n) mod 8 values, where n equal any integer in therange 0 to 7. If every waveform is properly counted, that relationshipis maintained indefinitely. Control of the number n provides theopportunity to adjust the baseline with a resolution of ⅛ of full scale.In example immediately above, the receiving transducer signal initiallystarts in the “2^(nd)” relationship with the transmitting transducersignal, (N+n) mod 8=1. By controlling n so that (N+n) mod 8=0, thebaseline is adjusted so the phase measurement range is expanded to itsmaximum extent and the determination of gas composition is maximized,and is a feature of the present invention.

For the phase shift detection described above, the electronic circuitryof the controller block 200 of the analyzer unit 100 is implemented bydigital circuits, according to an embodiment of the present invention.The oscillator block 201 generates signals at a fixed frequency. In thisembodiment the frequency is 40 KHz. The oscillator block signal drivesthe transmitting transducer 404 and is divided in frequency by 8, orstated differently, the counter 205 steps down, or lowers, the signalfrequency by a factor of 8. The output of the counter 205 is received bythe phase detector block 206.

The output of the receiving transducer 206 is processed into squarewaves by comparative logic (block 203) after being amplified by theamplifier 202. The counter 204 divides the signal frequency by 8. Theoutput of the counter 204 is also received by the phase detector block206. Through the operation of an Exclusive-OR or an Exclusive-NOR gate,the phase detector 206 output varies between the two power levels of thelogic gate, say, 0 and 5 volts, for example. The low-pass filter 207eliminates the AC component of the output signal.

Control of the continuous sonic wave analyzer unit 100 is performed bythe control unit 208 in the controller/analysis block 200. In thisembodiment the control unit 208 is basically a programmed microprocessoror microcontroller with memory. Among other contents, the memory storesvalues from the filtered phase detector 206. Control lines from the unit208 extend to each of the valves 401-403 and the counters 205 and 204.The unit 208 also receives phase shift values from the output of thelow-pass filter 207. A display 209 is connected to the control unit 208provides a visual interface for the operations of the analyzer unit 100.

Under the control unit 208, the analyzer unit 100 with an expanded phaseshift range determines the speed of sound in multiple gases and thecomposition of gases. The following description refers to the productionof ozone and to the FIG. 1 system, but the analyzer unit should not beconsidered so limited. Briefly stated, ambient air, is introduced intothe conduit and the valves closed. The output of the filtered phasedetector is read and recorded for ambient air. Then feed gas,oxygen-enriched air, is introduced into the conduit and the valvesclosed. The output of the filtered phase detector is read and recordedfor the feed gas. Finally ozone-bearing gas is introduced into theconduit and the valves closed. The output of the filtered phase detectoris read and recorded for ozone-bearing gas.

FIG. 5 shows a process flow of the operation described immediatelyabove. The steps of the process flow are generalized in that the gasesare labeled A, B and to indicate that more gases derived from theinitial gas may be included in this process flow. After the system isinitialized as represented by the dotted arrow 501, step 502 initializesan index Valve # to zero. Then the valve indicated by Valve # is openedand the gas selected by the opened valve is fed into the conduit by step503. (In the FIG. 1 ozone generation system, index Valve #=0 correspondsto value 403, index Valve #=1 corresponds to value 402, and index Valve#=2 corresponds to value 401). Step 504 closes the valve. Step 505 testswhether the index Valve # is zero or not. If the index is 0, thecounters are reset and the output value of the filtered phase detectoris read and recorded by step 507. If the index is not 0, the test ofstep 505 moves to step 507. After step 507 the index Valve # is testedwhether it is equal to 2 by step 508. If not, then step 509 incrementsthe index Valve # by 1 and the process returns to step 503. The stepsare repeated until index Valve # is equal to 2 and the process ends bystep 510.

From the recorded phase shift values, the control unit 208 analyzes thedata to determine the speed of sound and composition of the gases. Acomparative technique is used. With the speed of sound and compositionof the first gas already known, dehumidified ambient air is used as areference to determine the speed of sound and composition of gasesderived from the first gas, the ambient air. In particular, the speed ofsound of the dehumidified ambient air at the set temperature of thetemperature-controlled conduit is known and used as a reference tocalculate the speed of sound of the second gas, oxygen-enriched feedgas, and of the third gas, the enriched air bearing ozone, from themeasured phase shifts.

The gases are processed in the order of decreasing speed of sound, i.e.,ambient air, ambient air enriched with oxygen, and oxygen-enriched airbearing converted ozone. The enriched air is derived from the ambientair and the ozone-being air is derived from the enriched air. Each gasis more dense than the gas preceding it, and the speed of sounddecreases relative to the speed in previous gas(es). The first gas (dryambient air) is treated as a reference gas because its composition isknown. Its speed of sound at a given temperature is also known. While itis possible to allow the temperature to vary in the manner described inthe previously described U.S. Pat. No. 5,644,070 (Gibboney), it ispreferable that the temperature of the gases be maintained at a settemperature. The temperature-controlled gas conduits, such asillustrated in FIG. 1 , have been found effective at maintaining gasesat a set temperature. Thus in calculating the speed of sound in theoxygen-enriched air and ozone-bearing air, it is assumed that all gaseshave the same temperature. The addition of thermal mass or control ofthe environmental temperature is desirable in order to minimize error.With the speed of sound being the greatest in the reference first gas,the baseline may be adjusted as described earlier so that the phaseshift ranges of the oxygen-enriched air and ozone-bearing air areexpanded to accommodate the compositions of those gases.

The sound propagated in oxygen-enriched air arrives at the secondtransducer a little later than for ambient air. The additional delay ismeasured by the phase shift in combination with the known conduit lengthL between the first and second transducers. This allows determination ofthe speed of sound of the oxygen-enriched air as a function of speed ofsound of ambient air and the additional phase shift. As described above,there is a direct relationship between the phase shift and delay. Inparticular, with L=length of sound path, S0=known speed of sound inambient air, then delay D0 of the ambient air is D0=(L/S0), a knownquantity. The delay D1 of oxygen-enriched air is D0+Dx, where Dx is theadditional delay due to slower speed of sound in the oxygen-enriched airand is known from the additional phase shift for the oxygen-enrichedair. Hence S1=L/D1=L/(D0+Dx). With L, D0 and Dx known, S1 is determined.

The speed of sound of the ozone-bearing air is measured in the samefashion. With S0=known speed of sound in the ambient air, S1=speed ofsound in the oxygen-enriched air, and S2=speed of sound in theozone-bearing air. The delay D2 of ozone-bearing air is D0+Dz, where Dzis the additional delay due to slower speed of sound in theozone-bearing air and is known from the additional phase shift for theozone-bearing air. Hence S2=L/D2=L/(D0+Dz). With L, D0 and Dz known, S2is determined.

The speed of sound S0 of ambient air, which has an oxygen composition of20.9%, is known and the speed of sound of 100% oxygen is also known. Themeasured speed of sound S1 of the oxygen-enriched gas should fallbetween the two known speed of sound values as a proportion of oxygenrepresenting a mixture of the two gases, ambient air and 100% oxygen.This proportion may be calculated: proportion O₂=(S1−S0)/(S_(ox)−S0)where S_(ox) is the speed of sound in 100% oxygen gas. This discussionavoids many complex factors. The theoretical speed of sound in a gas cancalculated from many models, which in turn have many factors, includingBoltzmann's constant, temperature, mass of a molecule, and adiabaticconstant (which is not the same for all gases under discussion),discourage theoretical certainty. Fortunately, certain assumptions oflinearity yield reasonable approximations in the regions of interest.Empirical scaling yields good results.

Hence it has been found that: percentage O₂=79.1×(proportionO₂×O₂scale)+20.9 is a very good approximation. Because this model isapproximate and small variations will occur in the real world, thescaling factor O₂scale, near unity, is used to proportion O₂.

Similarly, for the ozone-bearing air: proportionO₃=O₃scale×3.329×(S1−S2)/(calculated speed of sound in pure ozone). Thespeed of sound in pure ozone is calculated because there is likely noway of empirically determining that speed of sound at ordinarytemperatures due to the explosively unstable nature of such a gas. Thenumber used is by calculation based upon molecular weight, temperature,and adiabatic constant. O₃scale is an empirical scale adjustment havinga value of near unity.

To determine the concentration of ozone in terms of amount of ozone percubic centimeter the following equation may be used: grams of ozone percm³=proportion O_(3l×2142.8571.)

Correction factors arise from complexities in the measurement of gascomposition. The production of ozone may not depend on dilution of onegas by another. For example, if 1 mole of oxygen passes through anelectrical discharge ozone cell and 10% of the O₂ is converted to O₃,the emerging ozone from the cell is 0.666 . . . mole due to thereduction in the number of molecules from the conversion from O₂ to O₃.The total oxygen emerging is 0.9 mole. Hence the total emerging gas is0.9666 . . . mole and the molar percentage O₃ is 6.9%. But the speed ofsound still has a 1-to-1 relationship to the ozone concentration.

If the gas entering the discharge cell consists of more than onecomponent, the situation is similar. For example, if the gas enteringthe cell is 90% O₂ and 10% N₂ by molar measure, 1 mole of gas consistsof 0.9 mole O₂ and 0.1 mole N₂. With 10% of the oxygen converted to O₃,the emerging gas consists of 0.06 mole O₃, 0.81 mole O₂, and 0.1 mole N₂for a total of 0.91 mole. The molar percentage O₂ is 6.2%. The speed ofsound still has a 1-to-1 relationship because each gas has aconcentration that is a unique function of the ozone concentration, andhence has a unique speed of sound corresponding to that concentration.

The change in the speed of sound depends upon the change in ozoneconcentration in a gas. In ozone generation systems, the oxygen contentin air is typically increased before the resulting gas is sent to theozone generation cell. Hence it is good to know the composition of thegas entering the generation cell, as well as after the cells. Air, forexample, can be assumed to be 78% N₂, 21% O₂ and 1% argon. The publishedspeed of sound at 0° C. is respectively 337 m/s, 316 m/s, and 307 m/sthrough these respective component gasses. By averaging these speeds inproportion to their proportion in air, an overall speed of sound in airis found to be 332 m/s. This compares well to a published speed of 331m/s. A reasonable estimate for the speed of sound in ozone at 0° C. is249 m/s, though this is an unlikely direct measurement since highconcentrations of ozone are unstable.

The following are illustrative examples with different concentrations ofoxygen entering the discharge cell. The first illustration assumes thata sample of gas consisting of 0.8 mole N₂ and 0.2 mole O₂, anapproximation of air. Following the calculations above, the speed ofsound in this mixture is 332.80 m/s. If this sample is then passedthrough an ozone generating cell, some of the O₂ is converted to O₃which reduces the total molar quantity of gas. Assuming that 0.1 mole ofthe O₂ is converted to O₃, the total output is 0.8 mole N₂, 0.1 mole O₂,and 0.0667 mole O₃, for a total of 0.967 mole. The molar percentage ofO₃ is 6.9%. The speed of sound in the mixture of gases is 328.65 m/s andthe change in speed of sound is −4.146 m/s with the proportional change−0.01245.

In comparison, with the assumption that the sample of gas consists of1.0 mole O₂, i.e., the sample is all oxygen, the speed of sound in thismixture is 316 m/s following the calculations above. If the sample ispassed through an ozone generating cell, some of the O₂ is converted toO₃ to reduce the total molar quantity of gas. Assume, as in the lastcase, that 0.1 mole of the O₂ is converted to O₃. The total output is0.9 mole O₂, and 0.0666 mole O₃, for a total of 0.967 mole. The molarpercentage of O₃ is 6.9% as before. The speed of sound in the mixture ofgases is 311.28 m/s. The change in speed of sound is −4.720 m/s and theproportional change is −0.01493.

It should be noted that the proportional change of the speed of soundrelative to proportion of oxygen in the feed gas entering the cell isgreater for pure oxygen than for air. This may be viewed in heuristicfashion. The proportion of nitrogen in the ozone-bearing gas increases,as oxygen is converted to ozone, if there is a lot of nitrogen to beginwith. The increased nitrogen, having a relatively high speed of sound,tends to counteract the reduction in the speed of sound due toincreasing proportion of ozone.

In returning to the conduit path length L, there should be consideredsome practical constraints to the length of the conduit as mentionedearlier. These constraints depend on the speed of sound of the gasesbeing measured, the frequency of operation, and the method of measuringor detecting phase shift. For each gas, there is a corresponding speedof sound and a corresponding wavelength. Among the gases there is a gaswith a maximum wavelength λ_(max) and a gas with a minimum wavelengthλ_(min). If phase shift is to be limited to 360 degrees, thenL/λ_(min)−L/λ_(max) must be less than 1, i.e.:L/λ _(min) −L/λ _(max)<1.

This corresponds to the difference in number of wavelengths contained inthe conduit is less than 1. By manipulating the terms to determine theconduit length, one obtains:L<λ _(min)·λ_(max)/(λ_(min)−λ_(max)).

Similarly, if phase shift is to be limited to 180 degrees, thenL/λ_(min)−L/λ_(max) must be less than ½, i.e.,L/λ _(min) −L/λ _(max)<½; orL<½λ_(min)·λ_(max)/(λ_(min)−λ_(max)) in this case.

Some exemplary numbers may illustrate these points. With the frequencyof the analyzer fixed at 40 KHz, and assuming that the speed of sound ofthe reference gas (air) S_(ref) is 343 m/s or λ_(ref)=0.858 cm, and therange of speed of sound of other gases (oxygen-enhanced air and ozone)in the analyzer, ΔS, is 290 m/s, the maximum wavelength λ_(max)(λ_(ref)) is 0.858 cm and the minimum wavelength λ_(min) is 0.725 cm. Ifphase shift is to be limited to 180 degrees, thenL((1/0.725)−(1/0.858))<½, or L<2.339 cm. Similarly, if phase shift is tobe limited to 360 degrees, then L<4.677 cm. These numbers correspond to2.726 wavelengths of the reference gas (λ_(ref)) and 5.452 wavelengthsof the reference gas (λ_(ref)) respectively.

But very short conduits cause artifacts due to standing waves, artifactsdue to the acoustic contribution of holes for ingress and egress of testgases, artifacts due to the uncertain phase relationship of electricalsignals to acoustic signals, and artifacts due to electrical and/oracoustical noise. For these reasons, it is desirable to have a conduitthat includes at least 10 wavelengths of sound in the reference gas inorder to minimize these artifacts. As described in the previousparagraph, a conventional phase detector places a severe constraint onconduit length. In the case of a 360° phase shift detector, path lengthL of the conduit in terms of number of wavelengths of the reference gascan be no greater than 5.452, and in the cases of a 180 degree phaseshift detector, the path length can be no more 2.726 wavelengths of thereference gas.

However, by the application of the previously described frequencydivision technique upon the particular phase shift detection method, theconstraint on the conduit path length L can be removed and L lengthened.If the frequency is divided by n, n=8 for example, the maximum number ofwavelengths and the maximum length L are each multiplied by a factor ofn=8. That is, the upper bound for the conduit path length becomes:L<λn _(min)·λ_(max)/(λ_(min)−λ_(max)) or L<½nλ_(min)·λ_(max)/(λ_(min)−λ_(max))depending upon whether the phase shift detector is 360 degrees or 180degrees respectively.

On the other hand, even with the frequency division technique the upperbounds of the conduit path length are not limitless. Long conduits alsocause problems which include signal attenuation, high sample volume, andbulky design. It is desirable to limit the conduit length L to about 30wavelengths, at which point the disadvantages of long path length beginto become severe.

In a preferred embodiment, a length of conduit corresponding to about 23wavelengths of sound in reference gas (λ_(ref)) was selected, i.e., Lapproximately =23*0.858 cm. This creates a situation in which phaseshift exceeds the limit of the 180° phase detector selected, but the useof the frequency division technique described maintains the advantagesof a relatively long signal path.

The described relatively simple and low-cost gas analyzer measures theconcentration of ozone in an ozone generation system with highresolution and precision, and relatively independently of the oxygenconcentration in the feed gas and independently of temperature.Additionally, the concentration of oxygen in the gas fed to the ozonegeneration is precisely measured. This provides an inexpensive andproductive way of generating ozone on site and at the time of use.

This description of the invention has been presented for the purposes ofillustration and description. It is not intended to be exhaustive or tolimit the invention to the precise form described, and manymodifications and variations are possible in light of the teachingabove. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical applications.This description will enable others skilled in the art to best utilizeand practice the invention in various embodiments and with variousmodifications as are suited to a particular use. The scope of theinvention is defined by the following claims.

The invention claimed is:
 1. An analyzer for one or more gases derivedfrom a first gas of known composition and speed of sound, each derivedgas having a concentration of a component changed, the analyzercomprising: a first transducer, the first transducer driving continuoussound waves responsive to a fixed frequency signal source; a conduitacoustically connected to the first transducer, the conduit selectivelyreceiving and holding samples of the first gas and one or more derivedgases; a second transducer acoustically connected to the conduitopposite the first transducer, the second transducer receiving soundwaves from the first transducer through the conduit and generatingsecond transducer signals responsive to the received sound waves; aprocessing unit receiving fixed frequency signal source signals and thesecond transducer signals, the processing unit determining a relativephase shift between the frequency source signals and second transducersignals for a gas sample in the conduit, the relative phase shiftcorresponding to a difference of speed of sound in one gas samplerelative to another gas sample, the processing unit including circuitrylowering the frequency of the received fixed frequency source signalsand second transducer signals to expand the range of measurement of therelative phase shift; and a calculating unit determining from the firstgas of known composition the speed of sound of the one or more gasesderived from the first gas, and calculating the composition of a sampleof one or more derived gases from the first gas as a reference.
 2. Theanalyzer of claim 1 wherein the circuitry lowering the fixed frequencyof the frequency source signals and second transducer signals eachcomprises a frequency divider circuit.
 3. The analyzer of claim 2wherein the frequency divider circuit comprises a digital countercircuit.
 4. The analyzer of claim 3 wherein the frequency dividercircuit divides by a divisor having a value of a power of
 2. 5. Theanalyzer of claim 4 wherein the frequency divider circuit divides by 8.6. The analyzer of claim 3 wherein the digital counters of the frequencydividers have baselines adjusted so that the range of measurement of thephase shift is expanded and the range of calculation of gas compositionis increased.
 7. The analyzer of claim 2 wherein the conduit has a pathlength L between the first and second transducers wherein L has aremoved constraint from the frequency divider circuit of:L<λ _(min)·λ_(max)/(λ_(min)−λ_(max)) orL<½λ_(min)·λ_(max)/(λ_(min)−λ_(max)) depending upon whether theprocessing unit limits the relative phase shift to 360 degrees or to 180degrees respectively, where λ_(min)=minimum wavelength andλ_(max)=maximum wavelength of the first gas and one or more gases. 8.The analyzer of claim 7 wherein the conduit has a path length L:L<nλ _(min)·λ_(max)/(λ_(min)−λ_(max)) or L<½nλ_(min)·λ_(max)/(λ_(min)−λ_(max)) depending upon whether the processingunit limits the relative phase shift to 360 degrees or to 180 degreesrespectively, and n comprises a divisor for the frequency dividercircuit.
 9. The analyzer of claim 8 wherein the conduit path length L ismore than 10 wavelengths of the first gas.
 10. The analyzer of claim 9wherein the conduit path length L is less than 30 wavelengths of thefirst gas.
 11. The analyzer of claim 1 wherein the first gas comprisesair.
 12. The analyzer of claim 11 wherein a first derived gas comprisesair with an increased concentration of oxygen.
 13. The analyzer of claim12 wherein a second derived gas comprises air with an increasedconcentration of ozone.
 14. The analyzer of claim 1 wherein thecalculating unit calculates the concentration of the component changedin the sample of the one or more derived gases.
 15. The analyzer ofclaim 14 wherein the concentration of the component changed in thesample of the one or more derived gases is calculated principally fromthe proportion of the changed component in the sample of the one or morederived gases.
 16. The analyzer of claim 15 wherein the proportion ofthe changed component in the sample of the one or more derives gases iscalculated from a ratio having the determined speed of sound of thesample of the one or more derived gases in the numerator and a speed ofsound of 100 percent of the changed component in the denominator. 17.The analyzer of claim 15 wherein the calculation of the concentration ofthe component changed in the sample of the one or more derived gasesincludes modifying empirical correction factors.
 18. The analyzer ofclaim 1 wherein the conduit is maintained at a selected temperature. 19.The analyzer of claim 18 wherein the conduit comprises anozone-resistant tubing enclosed in a metal block to provide a thermalmass to the conduit.
 20. The analyzer of claim 19 wherein theozone-resistant tubing comprises polytetrafluoroethylene (PTFE) and themetal block comprises aluminum.
 21. A method of operating an analyzerfor one or more gases derived from a first gas of known composition andspeed of sound, each derived gas having a concentration of a componentchanged, the method comprising: driving continuous sound waves with afirst transducer in response to fixed frequency electrical signalsthrough a conduit holding a sample of the first gas or one or morederived gases at a time; receiving the sound waves driven through theconduit by a second transducer and generating electrical signals inresponse to the received sound waves, a phase shift between the receivedsound wave signals and the driven sound wave signals corresponding to aspeed of sound in the gas sample; processing the fixed frequencyelectrical signals and the electrical signals generated signals inresponse to the received sound waves at a lowered frequency to expandthe range of measurement of the phase shift to determine a relativephase shift between the frequency source signals and second transducersignals for a gas sample in the conduit, the relative phase shiftcorresponding to a difference of speed of sound in one gas samplerelative to another gas sample; determining a speed of sound of one ormore gases derived from the first gas from the known speed of sound ofthe first gas and from the relative phase shift of gas samples of thefirst gas and one or more gases derived from the first gas in theconduit, and calculating a composition of a gas sample of one or moregases derived from the first gas as a reference.
 22. The method of claim21 wherein the processing step includes lowering the frequency of thefixed frequency electrical signals and the electrical signals generatedsignals in response to the received sound waves.
 23. The method of claim22 wherein the frequency lowering step comprises dividing the frequencyby divisor having a value of a power of
 2. 24. The method of claim 23wherein the frequency lowering step comprises dividing the frequency by8.
 25. The method of claim 21 wherein the first gas comprises air, afirst derived gas comprises air with an increased concentration ofoxygen, and a second derived gas comprises air with an increasedconcentration of ozone.
 26. The method of claim 21 wherein thecalculating step comprises calculating the concentration of thecomponent changed in the sample of the one or more derived gases. 27.The method of claim 26 wherein the component changed calculating stepcomprises calculating the concentration of the component changed in thesample of the one or more derived gases principally from the proportionof the changed component in the sample of the one or more derived gases.28. The method of claim 27 wherein the component changed calculatingstep comprises calculating the proportion of the changed component inthe sample of the one or more derived gases from a ratio having thedetermined speed of sound of the sample of the one or more derived gasesin the numerator and a speed of sound of 100 percent of the changedcomponent in the denominator.
 29. The method of claim 27 wherein thecomponent changed calculating step includes modifying the proportion ofthe changed component in the sample of the one or more derived gaseswith empirical correction factors.
 30. The method of claim 21 furthercomprising maintaining the conduit at a selected temperature.
 31. Themethod of claim 21 further comprising providing for a conduit having apathlength of greater than 10 wavelengths of sound in air.
 32. A methodof determining the composition of one or more gases derived from a firstgas of known composition and speed of sound, the method comprising:driving continuous sonic waves through the conduit at a fixed frequency,a phase difference between sonic waves entering a conduit and leavingthe conduit corresponding to a speed of sound of a gas in the conduit;processing electronic signals corresponding to the continuous sonicwaves entering the conduit and leaving the conduit at a loweredfrequency to expand the range of measurement of the phase shift;changing the gas in the conduit among the first gas and the one or morederived gases; determining the speed of sound of the first gas of knowncomposition and the one or more gases derived from the first gas in theexpanded range from a relative phase shift of gases of the first gas ofknown composition and the one or more derived gases, the relative phaseshift corresponding to a difference of speed of sound in one gas samplerelative to another gas sample; and calculating a composition of the oneor more derived gases from the first gas as a reference.
 33. The methodof claim 32 wherein each derived gas has a concentration of a componentchanged originally from the first gas.
 34. The method of claim 32wherein the processing step includes lowering the frequency of thecontinuous sonic waves entering the conduit and leaving the conduit at alowered frequency to expand the range of measurement of the phase shift.35. The method of claim 34 wherein the frequency lowering step comprisesdividing the frequency by a divisor having a value of a power of
 2. 36.The method of claim 35 wherein the frequency lowering step comprisesdividing the frequency by a divisor equal to
 8. 37. The method of claim32 wherein the first gas comprises air, a first derived gas comprisesair with an increased concentration of oxygen, and a second derived gascomprises air with an increased concentration of ozone.
 38. The methodof claim 32 wherein the calculating step comprises calculating theconcentration of the component changed in the sample of the one or morederived gases.
 39. The method of claim 38 wherein the component changedcalculating step comprises calculating the concentration of thecomponent changed in the sample of the one or more derived gasesprincipally from the proportion of the changed component in the sampleof the one or more derived gases.
 40. The method of claim 39 wherein thecomponent changed calculating step comprises calculating the proportionof the changed component in the sample of the one or more derived gasesfrom a ratio having the determined speed of sound of the sample of theone or more derived gases in the numerator and a speed of sound of 100percent of the changed component in the denominator.
 41. The method ofclaim 39 wherein the component changed calculating step includesmodifying the proportion of the changed component in the sample of theone or more derived gases with empirical correction factors.
 42. Themethod of claim 32 further comprising maintaining the conduit at aselected temperature.
 43. The method of claim 32 further comprisingproviding for a conduit having a path length of greater than 10wavelengths of sound in air.